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Colliding satellites stress crowded skies

The collision between two communications satellites yesterday highlight just how crowded the heavens are getting. At about noon Washington time, a US

12 February 2009/9:35GMT

The collision between two communications satellites yesterday highlight just how crowded the heavens are getting.

At about noon Washington time, a US Iridium phone satellite piled into a defunct Russian relay station, about 500 miles above Siberia. Both satellites were destroyed, releasing debris that may even threaten the International Space Station, orbiting 220 miles above the Earth.

NASA scientist Nicholas Johnson said this was the first time two such large man-made items had collided in space and could not rule out the risk that their collision could threaten other satellites.

Wikipedia estimates there are around 3000 satellites orbiting the Earth, with the US Strategic Command monitoring about 15000 more man-made objects (mostly debris).

The risk of collision is particularly high with 'constellations' of satellites - networks of navigation or communications satellites that aim to provide global coverage from roughly the same altitude.

As far back as 1996, a paper published by the American Institute of Aeronautics and Astronautics calculated that two satellite constellations had a 50 percent chance of a collision every four or five years - with that time frame shrinking rapidly as the number of satellites increases.

Losing one satellite from the Iridium constellation won't have a huge impact on its sat-phone network, but the skies are about to get even busier, with the launch of new satellite navigation constellations by Europe (Galileo), Russia (GLONASS) and China (Beidou).

In short, enjoy your sat navs, sat phones and satellite radios while you can - the sky really could be falling in..